1. Field of the Invention
The present invention relates to distance measurement using radio frequency (RF) signals and in particular to incorporating a location symbol into the DATA field of a legacy packet, thereby facilitating refined distance measurement to within one meter.
2. Related Art
Wireless local area networking (WLAN) technologies based on the IEEE 802.11 set of standards transmit data packets via air. Determining the distance between nodes in a WLAN can be done by computing a round trip time of packets sent between the nodes and then determining air propagation time. An article authored by Andre Gunther and Christian Hoene, entitled “Measuring Round Trip Times to Determined the Distance between WLAN Nodes”, published in 2005 describes such a computation using a data/acknowledgement sequence in detail. This type of conventional round trip computation can provide distance accuracy within 8 meters.
U.S. Pat. No. 7,899,472, issued to Atheros Communications on Mar. 1, 2011 and incorporated by reference herein, describes other techniques that can be used to determine distance and position. For example, when associated with a WLAN, a wireless device can quickly determine its position based on information provided by an access point in the WLAN. Even when a wireless device disassociates with the access point, the WLAN can provide time, location, and decoded GPS data to the station before its disassociation. In this manner, the wireless device can significantly reduce the time to acquire the necessary GPS satellite data (i.e. on the order of seconds instead of minutes) to determine its coordinate position.
This technique can include identifying a first access point having a first known location and a predetermined radio propagation range of r. The position of a wireless device, which is capable of associating with the access point, is the first known location with an uncertainty of r. In this manner, even the relative position of the wireless device is within an acceptable range of accuracy for emergency service.
To further refine this relative position, the technique can further include determining a radio signal strength indicator (RSSI) of the access point. The RSSI can be correlated to a distance, which could be stored in a lookup table (LUT) in the wireless device. The stored distance represents a more accurate relative distance from the access point to the wireless device.
To obtain a coordinate position (x0, y0, z0) of the wireless device, the wireless device can receive information from additional access points. Specifically, the wireless device can identify second and third access points, each having known locations. The wireless device can then determine second and third RSSIs of the second and the third access points, respectively. The second and third RSSIs can be correlated to second and third stored distances. The second stored distance represents a second relative distance from the second access point to the wireless device, whereas the third stored distance represents a third relative distance from the third access point to the wireless device. At this point, the wireless device can compute its coordinate position using the first, second, and third known positions and the first, second, and third relative distances.
In yet another technique, the use of multiple antennas within an access point can also facilitate quickly and accurately providing a coordinate position of the wireless device. In this technique, the wireless device can determine an RSSI associated with each of the multiple antennas. The signal path of the antenna associated with a highest RSSI can be identified. Once again, the highest RSSI can be correlated to a stored distance, wherein the stored distance represents a relative distance from the access point to the wireless device. At this point, the wireless device can use the angles associated with the signal path and the relative distance to compute a coordinate position of the wireless device.
In general, the techniques described in U.S. Pat. No. 7,899,472 can provide distance/position information with an accuracy of 2-3 meters. However, a need arises for even greater accuracy, i.e. within one meter.